HOW FAR HAS THE FAILED ANZA RIFT PROPAGATED INTO AFRICA?

The Turkana depression is a NW-trending narrow topographic corridor within the East African Rift System between the southern Main Ethiopian Rift and the Kenyan – Turkana rifts in northern Kenya, southern Ethiopia and eastern South Sudan. The corridor is dominated by outcrops of Precambrian basement rocks, Mesozoic-Cenozoic rifts and Cenozoic volcanic rocks. Physiographically, the Turkana depression is bounded in the northeast by the Ethiopian-Yemen plateau and to the southwest by the East African plateau. The Anza rift within the Turkana depression in Kenya is considered a NW- trending failed arm of the paleo-triple junction (now represented by the Lamu embayment) that separated the African continent from Madagascar during the Jurassic. It is part of a Mesozoic rift system that extends in eastern, central and western Africa. The similarity of Cretaceous structures developed within the Sudan – South Sudan rifts and the Anza rift is suggestive of the connection of the two rifts across the Turkana depression. However, upper crustal geophysical imaging has only been able to document the northwestward continuation of the Anza rift to the Turkana rift which is ~600 km to the southeast of the Sudan - South Sudan rifts. In order to examine any crustal scale linkage between the Anza rift and the Sudan – South Sudan rifts, we modeled the crustal thickness variation by using three-dimensional (3D) gravity forward modelling and two-dimensional (2D) gravity inversion of the European Improved Gravity Model of the Earth by New Techniques (EIGEN – 6C4). Our results show systematic thinning of the crust, in the range of 24-28 km, beneath the strike of the Turkana depression along which the Anza rift and the Sudan - South Sudan rifts trend. Provided that the shallowing of the Moho beneath the depression is related to the Mesozoic rifting process, it is possible to conclude that the Anza rift connects the Lamu embayment paleo-triple junction with the Sudan – South Sudan rifts. Since Mesozoic rifts in South Sudan have been found petroliferous, this finding is complementary to the exploration of petroliferous rifts in Kenya. In addition, it provides a new dimension that highlights the influence of the NW-trending pre-existing structures in the evolution of the NE-trending segments of the East African Rift System.